1. Field of the Invention
The present invention relates to a clamp for protecting an integrated circuit against voltage surges which may occur between two terminals of the integrated circuit.
Voltage surges applied between two terminals of an integrated circuit can destroy the circuit. These voltage surges most often result from electrostatic discharges which can be inadvertently applied to the terminals of the integrated circuit in various circumstances. These circumstances may be a mere manipulation of the circuit by an operator whose fingers are brought to high electrostatic potentials by friction phenomena.
2. Discussion of the Related Art
FIG. 1 shows a conventional clamp 12. This device includes a bipolar NPN shunt transistor Q1 connected by its collector to terminal V.sup.+ and by its emitter to terminal V.sup.-. One or more Zener diodes 14 may be used to define the limit voltage between terminals V.sup.+ and V.sup.-, beyond which transistor Q1 turns on. The anode voltage of the Zener diodes 14 is applied to the base of transistor Q1 through a bipolar NPN follower transistor Q2. The bases of transistors Q1 and Q2 are connected to terminal V.sup.- through respective resistors 18 and 19.
With this configuration, transistor Q1 is turned on when the voltage between terminals V.sup.+ and V.sup.- becomes higher than the value Vz+2Vbe, where Vz is the sum of the voltages of Zener diodes 14 and 2Vbe is the sum of the base-emitter voltages of transistors Q1 and Q2.
Transistor Q1 is chosen with a particularly low on-resistance. It has a size of, for example, 300 elementary transistors. Follower transistor Q2 is used to supply a base current sufficient for transistor Q1 and includes for this purpose, for example, 100 elementary transistors.
A drawback of this clamp is the difficulty in obtaining a well known Zener voltage Vz. This Zener voltage must be higher than the nominal supply voltage of the circuit but must be lower than the maximum admissible voltage before circuit breakdown.
This range is relatively narrow and it often happens that, from the construction, the Zener voltage obtained is too low so that the clamp starts operating as soon as the circuit is normally supplied. It also often happens that, during circuit operation, by mere thermal drift or aging, the Zener voltage decreases and activates the clamp while the circuit is normally supplied.
Another drawback of this clamp is that transistor Q1 can be destroyed by a long voltage surge, which, however, is insufficient to damage the circuit the clamp protects. As a result, transistor Q1 is permanently short-circuited, which makes the circuit inoperable, even though it is in working order.
Most prior art clamps used in fast technologies are implemented by means of bipolar transistors. An electrostatic discharge has an almost vertical voltage leading edge followed by a progressive decrease. It is essentially this initial voltage value that must be reduced. For this purpose, the clamp must be able to react almost instantaneously. Up to now, it has not been possible to obtain an efficient clamp in slow CMOS technologies (for example, HC1PA).